The present invention relates to an improvement of a radiographic intensifying screen (hereinafter referred to simply as an "intensifying screen").
The intensifying screen has a structure wherein a fluorescent layer comprising an X-ray luminescent phosphor i.e. a phosphor which is capable of instantaneously emitting a light of high luminance when excited by X-ray radiation (hereinafter referred to as "X-ray phosphor"), and a binder resin, is formed on a support made of paper, a plastic or the like, and a thin plastic protective layer which is capable of transmitting a light, is formed thereon. It is used for medical diagnosis or for non-destructive inspection of materials to convert an X-ray image of an object to a visible image and record it on an X-ray photographic film.
For such an intensifying screen, it is known that if the content of the binder resin in the fluorescent layer is reduced (i.e. if the content of the X-ray phosphor is increased), the speed of the intensifying screen and the image quality, particularly the sharpness, will be correspondingly improved. However, X-ray phosphor particles are usually uniformly dispersed in the binder resin substantially throughout the entire area of the fluorescent layer. Accordingly, if the content of the binder resin in the fluorescent layer is reduced to improve the sharpness of the intensifying screen, the probability of the phosphor particles being exposed to the surface of the fluorescent layer becomes correspondingly high. If a protective layer made of a thin film separately formed is bonded to the surface of such a fluorescent layer to produce an intensifying screen, no adequate bonding strength will be obtained between the fluorescent layer and the protective layer, and there has been a problem that the protective layer is likely to be peeled from the fluorescent layer by repeated use.
It is an object of the present invention to provide an intensifying screen having a protective layer bonded on the surface of a fluorescent layer, whereby the sharpness is improved by reducing the content of the binder resin in the fluorescent layer to a level lower than the average content in conventional intensifying screens and the adhesive strength between the fluorescent layer and the protective layer is strengthened by unevenly distributing the binder resin in the fluorescent layer.
The above object of the present invention can be attained by reducing the content of the binder resin contained in the fluorescent layer of an intensifying screen to a level lower than the content of the binder resin in conventional usual intensifying screens and unevenly distributing the binder resin in the fluorescent layer towards the protective layer side so that the distribution of the binder resin in the fluorescent layer is relatively large at the protective layer side.
Namely, the present invention provides a radiographic intensifying screen which comprises a support, a fluorescent layer formed thereon comprising an X-ray excitable phosphor and a binder resin supporting the X-ray phosphor in a dispersed state, and a protective layer formed on the fluorescent layer, wherein the proportion of the binder resin to the X-ray phosphor is within a range of from 4 to 8% by weight on the average of the entire fluorescent layer, and the binder resin is unevenly distributed in the fluorescent layer so that the proportion of the binder resin to the X-ray phosphor in the fluorescent layer in the vicinity of the protective layer is higher than the proportion of the binder resin to the X-ray phophor in the remainder of the fluorescent layer.
Now, the present invention will be described in detail with reference to the preferred embodiments.
For the production of the intensifying screen of the present invention, firstly the X-ray phosphor and the binder resin are mixed, then a suitable amount of a solvent is added to prepare a coating dispersion of the phosphor having a proper viscosity. This coating dispersion of the phosphor is coated on a support made of e.g. paper or plastics such as polyester such as polyethyleneterephthalate, polystyrene, polyamide or polycarbonate by a roll coater or a knife coater. At this time, the viscosity, the drying rate, etc. of the coating dispersion of the phosphor are adjusted so that a part of the X-ray phosphor is sedimented prior to the solidification of the coated layer of the coating dispersion of the phosphor so as to form a fluorescent layer in which the content of the binder resin in the vicinity of the surface of the fluorescent layer is high as compared with the remainder of the fluorescent layer.
There is no particular restriction as to the X-ray phosphor to be employed for the intensifying screen of the present invention. Any conventional phosphor may be used which is capable of instantaneously emitting a light of high luminance when excited by X-ray radiation, including a tungstate phosphor such as M.sup.II WO.sub.4, M.sup.II WO.sub.4 :Pb (wherein M.sup.II is at least one member selected from the group consisting of Ca, M9, Zn and Cd), a rare earth oxyhalide phosphor or a rare earth oxysulfide phosphor represented by the formula M.sub.(w-n).M'.sub.n O.sub.w X (wherein M is at least one member selected from the group consisting of Y, La, Gd, and Lu, M' is at least one member selected from the group consisting of Ce, Dy, Er, Eu, Ho, Nd, Pr, Sm, Tb, Tm and Yb, X is sulfur or halogen, n is from 0.0002 to 0.2, and w is 1 when X is halogen and 2 when X is sulfur), an alkaline earth complex fluorohalide phosphor such as BaFX:Eu.sup.2+, (Ba,Me)FX:Eu.sup.2+, BaF.sub.2.pMeX.sub.2.qKX'.rM'eSO.sub.4 :rEu.sup.2+ (wherein Me is at least one member selected from the group consisting of Mg, Ca and Sr, M'e is at least one member selected from the group consisting of Ca, Sr and Ba, each of X and X' is at least one member selected from the group consisting of Cl and Br, p, q and r are numbers satisfying 0.80.ltoreq.p.ltoreq.1.5, 0.ltoreq.q.ltoreq.2.0 and 0.ltoreq.r.ltoreq.1.0, respectively), a phosphate phosphor such as LnPO.sub.4 :Tb (wherien Ln is at least one member selected from the group consisting of La, Y and Gd), (Ba,Sr).sub.3 (PO.sub.4).sub.2 :Eu.sup.2+ and HfP.sub.2 O.sub.7 :Cu, a tantalate phosphor and a niobate phosphor represented by the formula Ln'(Ta.sub.1-x,Nb.sub.x)O.sub.4 :Tm or Ln'(Ta.sub.1-x,Nb.sub.x)O.sub.4 :Tb (wherein Ln' is at least one member selected from the group consisting of Y, Gd and Lu), a sulfide phosphor such as ZnS:Ag or (Zn,Cd)S:Ag, a sulfate phosphor such as BaSO.sub.4 :Pb or (Ba,Sr)SO.sub.4 :Eu.sup.2+, and an iodide phosphor such as CsI:Tl, NaI:Tl or KI:Tl.
As the binder resin constituting a part of the fluorescent layer in the intensifying screen of the present invention, those commonly employed for the fluorescent layers of conventional intensifying screens may be used alone or in combination as a mixture of two or more different kinds. Specifically, the binder resin may be cellulose nitrate, cellulose acetate, ethyl cellulose, polyvinyl acetate, polyvinyl butyral, linear polyester, a vinyl chloride-vinyl acetate copolymer, polycarbonate, polyurethane, polymethyl methacrylate, cellulose acetate butyrate, polyvinyl alcohol or a mixture thereof.
The solvent useful for the preparation of the coating dispersion of the phosphor may be an alcohol such as methanol, ethanol, propanol or butanol, a ketone such as acetone, methyl ethyl ketone or cyclohexanone, an ether such as methyl ethyl ether or ethyl ether, an ester such as ethyl acetate or butyl acetate, xylene or a mixture thereof. Among them, it is particularly preferred to employ a solvent having a relatively slow drying property such as cyclohexanone, butanol, butyl acetate or xylene.
The coating dispersion of the phosphor is prepared by mixing the above X-ray phosphor, the binder resin and the solvent, further adding, if necessary, a dispersant such as phthalic acid, stearic acid or a lipophilic surfactant and a plasticizer such as a phospholic acid ester such as triphenyl phosphate, a phthalic acid ester such as diethyl phthalate or glycolic acid ester such as butylphthalylbutyl glycolate, which are commonly used as additives for the formation of fluorescent layers of conventional intensifying screens, and thoroughly mixing the mixture. The mixing ratio of the X-ray phosphor and the binder resin may be optionally changed. However, in order to obtain adequate adhesive strength between the fluorescent layer and the protective layer, the proportion of the binder resin to the X-ray phosphor is preferably at least 4% by weight on the average of the entire fluorescent layer even under the condition that the binder resin is unevenly distributed to the protective layer side of the fluorescent layer. Further, in order to improve the sharpness of the resulting intensifying screen, it is preferably at most 8% by weight on the average of the entire fluorescent layer. Further, it is preferred to control the amount of the solvent to bring the viscosity of the coating dispersion of the phosphor to a level of not higher than about 7,000 cP i.e. lower than the viscosity of a conventional phosphor coating dispersion, so that when the coating dispersion of the phosphor is coated on a support, a part of phosphor particles is sedimented before the solution is completely dried and solidified so that when dried, the content of the binder resin in the vicinity of the surface of the fluorescent layer is higher than that of the remainder of the fluorescent layer.
To dry the coating dispersion of the phosphor coated on the support, it is preferred to conduct the drying by taking a sufficient time, for instance, by gradually heating from room temperature while controlling the substitution of the ambient air without adopting rapid drying such as blowing hot air immediately after the coating. Particularly when a coating dispersion of the phosphor with a viscosity being not so low is coated on a support, it is necessary to control the drying rate of the phosphor coating solution so that during the drying process of the coating dispersion of the phosphor, phosphor particles are permitted to sediment to a proper extent before the phosphor coating dispersion is completely solidified.
For forming on a support a fluorescent layer in which a binder resin is unevenly distributed so that it is present in a relatively large amount at the protective layer side, during the preparation of the intensifying screen of the present invention, it is possible to employ a method of controling the viscosity of the coating dispersion of the phosphor or the drying rate of the coating solution on the support, as mentioned above, or to employ a method wherein two types of coating dispersions of the phosphors differring in the content (to the weight of the X-ray phosphor) of the binder resin (the average value of the binder resin contents of the respective coating dispersions of the phosphors being within a range of from 4 to 8% by weight to the X-ray phosphor) are preliminarily prepared, and the coating dispersion of the phosphor having a low binder resin content is firstly coated and dried on a support, and the coating dispersion of the phosphor having a high binder resin content is coated and dried thereon. When the fluorescent layer is prepared by this latter method, the solvent for the coating dispersion of the phosphor is not required to be of a slow drying type and may freely be selected among known solvents. Needless to say, the coating dispersion of the phosphor may be prepared at an optional viscosity, and it is unnecessary to control the drying rate of the coating dispersion of the phosphor coated on the substrate.
Irrespective of which one of the above methods is employed for the preparation of the fluorescent layer, the content (to the X-ray phosphor) of the binder resin in the vicinity of the surface of the fluorescent layer (the protective layer side) is preferably higher by more than 1.1 times than that of the remainder of the fluorescent layer, in order to maintain the adhesive strength between the protective layer and the fluorescent layer having a binder resin content of not higher than 8% by weight (to the X-ray phosphor), at a practically satisfactory level.
Then, a film of a synthetic resin such as polyethyleneterephthalate, polyvinylidene chloride, polyethylene or polyamide having an adhesive preliminarily coated, is bonded on the surface of the fluorescent layer formed on the support as described above, to obtain an intensifying screen of the present invention.
Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted by such specific Examples.